Researchers allowed beetle larva to feast on antibiotic-treated leaves and natural leaves and found that on the antibiotic-treated leaves, the beetles suffered from the plant's anti-herbivore defense, but on the natural leaves the larva gained more weight and thrived. (Credit: andriux-uk/Flickr)

Plants have different defenses to fend off both herbivores and pathogens.

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Plant defenses against chewing insects follow a jasmonite-mediated pathway that induces protease inhibitors and polyphenol oxidase, which suppress digestion and growth. Defenses against pathogens follow a salicylic acid mediated pathway. When the antimicrobial response turns on, it interferes with the response to chewing, allowing the beetles to develop more normally.

“For the last couple of decades, my lab has focused on induced defenses in plants,” says Gary W. Felton, professor and head of entomology at Penn State. “We had some clues that oral secretions of beetles suppressed defenses, but no one had followed up on that research.”

Seung Ho Chung, graduate student in entomology working with Felton, decided to investigate how plants identified chewers and how herbivores subverted the plants’ defenses.

“I thought we could identify what was turning the anti-herbivore reaction off,” says Felton. “But it was a lot more difficult because we had not considered microbes.”

Healthier and heftier

For the study published in the Proceedings of the National Academy of Sciences, Chung and Felton used tomato plants to identify exactly what was turning off the response to chewing. Colorado potato beetles also attack eggplant and potato plants.

They allowed beetle larva to feast on antibiotic-treated leaves and natural leaves and found that on the antibiotic-treated leaves, the beetles suffered from the plant’s anti-herbivore defense, but on the natural leaves the larva gained more weight and thrived.

Chung and Felton then investigated expression of genes in the anti-herbivore pathway and the production of enzymes. They found that the presence of bacteria decreases the anti-herbivore response.

The researchers also isolated and grew the bacteria from the Colorado potato beetle guts. They found 22 different types of bacteria, but only three types suppressed the anti-herbivore response.

During a variety of other experiments, they found that in all cases presence of the bacteria that could suppress the anti-herbivore response led to healthier beetles.

The researchers are now beginning to see if these bacteria are present in Colorado potato beetles all over the US and also in Europe.

The US Department of Agriculture and the National Science Foundation supported this work.